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VPLS & the Third Mile

We know plenty about the first mile's access loops and the second mile's aggregation routers, B-RASs, and DSLAMs. Yet, as consumer triple-play networks get deployed in earnest, the focus will soon begin to shift to the "third mile" – that area of the network in which a mesh of high-speed connectivity is required among the second mile's aggregation boxes and a variety of content servers, softswitches, and Internet routers.

This third mile of the network today is quite a mishmash of Sonet, ATM, WDM, and PDH (plesiosynchronous digital hierarchy). But given the undeniable trend toward multiservice IP, the third mile is heading inexorably towards one unifying solution: Ethernet.

Edge router vendors take note: Before long, each operator's third mile will be operated not as a series of point-to-point backhaul links, but as one big virtual private LAN service (VPLS). Whoever offers the winning VPLS solution may end up reaping the largest rewards from service providers selling, not to the enterprise, but to the consumer.

It was always rather inaccurate to refer to the access network as the first mile or the last mile (since the access network often runs to three miles in length), but it caught on for marketing purposes and even made its way into the standards, thanks to the IEEE's 802.3ah, Ethernet in the First Mile (EFM) spec. That led a number of analysts (including our own) to begin referring to central office aggregation gear as part of a "second mile." Again, not very accurate, but it stuck, regardless how far that central office actually was from a customer.

I hesitate add to this misnomenclature, but while writing the Heavy Reading report Telco Triple Play: The DSL Imperative, a finding arose I wasn't even looking for: The network behind the aggregation layer will face tremendous new pressures as triple play takes off, forcing some innovative thinking about how to effectively carry packetized voice, video, and data from thousands of customer sites to the carrier core.

The focus in this report was primarily on the first mile – which DSL flavor worked best where, whether DSLAMs or broadband loop carriers would rule this market, and how regional differences would affect the market's development. Talking with service providers and vendors over the past three months uncovered an increasing interest in looking farther back into the network to understand the implications of moving to triple play.

First and foremost, triple play represents a flood of IP traffic traversing the access network, upwards of 20-Mbit/s peak traffic per home to accommodate multiple set tops, broadband data voice, and perhaps an HDTV stream. Second, this traffic is truly multiservice, with very different latency, jitter, packet loss, and protection requirements. Third, it is bursty.

Today, ATM-based DSLAMs hand off OC3s or OC12s to an add/drop multiplexer, which carries it up to the edge of the packet core, handing it to a multiservice switch or router. This ATM-over-Sonet/SDH architecture works fairly well for broadband data, as these are both very mature protocols with low-cost interfaces at these rates. Add in IP video, however, and the economics take a turn for the worse. Most DSLAM manufacturers are already building in Gigabit Ethernet uplinks to carry video traffic, and most will be making a wholesale change to IP DSLAMs in the next year, putting all their broadband data and packet voice onto Ethernet for backhaul.

With an Ethernet/IP access architecture in place, the next question to ask is how best to carry it through the metro and regional networks. Because it's Ethernet, one might be tempted to believe simple Ethernet switches would suffice, but that isn't the case. Ethernet switches tend to fall down on the three key requirements of triple play: scale, quality of service, and protection. This is where VPLS comes in. VPLS as a service delivery layer offers its own triple play of values to the operator transforming its backhaul network:

  1. Scaleability. Ethernet is quite limited in its ability to scale to the proportions required in a residential triple-play network, where each DSLAM can serve hundreds of subscribers with data, voice, and two or three channels of switched IP video. VLANs simply don't cut it here over the long term. VPLS, however, has built-in scaleability, taking advantage of MPLS and its ability to support millions of labels.

  2. Quality of Service. Ethernet has no real bandwidth reservation technique that is truly end-to-end, nor any traffic engineering technique optimized for triple play. VPLS can guarantee end-to-end QOS and preserve that capability as it scales. Telcos will soon learn that customers know packet loss when they see it and will quickly complain when their video service doesn't perform. QOS will be essential in retaining customers lured away from cable or satellite.

  3. Protection. Ethernet's Spanning Tree and Rapid Spanning Tree work well enough for data services but are not up to the challenge when it comes to the mass rollout of IP video services. VPLS uses MPLS-based fast reroute or standby LSPs (label-switched paths) to ensure sub-50ms protection, which is key for both video and VOIP. In many cases, VPLS can be used as the primary protection layer of a network, allowing some operators to build Sonet/SDH-free backhaul networks if their traffic is purely IP-based.


We've already witnessed this in action, and all indications are this trend is just getting started. Witness:



VPLS and triple play can all be taken as just one more step in the Ethernetization of the network, from the CPE onward. Telco video forces the issue more quickly than any other service today because of its combination of huge bandwidth, massive scale, and mass-market price pressure – three things that have come together for Ethernet in the past, and will continue to drive it forward just about forever.

— Scott Clavenna, Chief Analyst, Heavy Reading

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